Installing plumbing is a necessary step in ground preparation before pouring a concrete slab foundation for any structure like a house or garage. It's important to have precise plans and follow them so that the plumbing you need is all in the correct position when you try to build the walls and eventually hook up the appliances that the plumbing services above that slab. Luckily, all that's required to lay the plumbing is a few trenches and some angled fittings.
Find the points on your blueprint where the plumbing will emerge from the finished, concrete slab, and measure the ground to find those precise points with the measuring tape. Mark the points and the path of the underground pipes with spray paint.
Dig trenches about 18 inches down and approximately 6 inches wide along all the marked, spray painted paths with a shovel.
Lay the horizontal pipe into the trenches, and at the point where they should come out of the ground, place the angled pipe fittings on them, angled upward toward the sky.
Cut lengths of pipe with a saw sufficient to attach to the other side of the angled fittings, come out of the ground, and stand at least 8 inches above the finished, concrete surface.
Attach the pipe lengths you cut in the previous step to the angled fittings using PVC glue.
Refill the trenches you dug and tamp the ground so that the only things visible are your vertical pipes coming out of the ground. Be sure refilling the trench doesn't knock the pipes over from being straight.